1. Field of the Invention
The present invention relates generally to devices and methods for certifying hard disks, and more particularly to certifying devices that simultaneously write and read information to and from the disk during the certification process.
2. Description of the Prior Art
A common challenge facing media manufacturers today is the rapidly increasing cost associated with the magnetic certification step in fabrication of hard disks. As a result of the accelerating number of tracks on the disk surface, testing time has increased rapidly.
Additionally, large capital expenditures for new testers, as well as high maintenance and operations costs are being incurred.
There are generally two types of magnetic disk certification testing devices in use today. The first type operates in a track-to-track stepping mode in which the write head first moves and settles on one track. After settling, a pattern is written on the track. Then, at least one disk revolution is required before the written pattern can be read back. In this type of device the drifting of the read/write head between the write revolution and the read revolution must be carefully controlled. This control is currently becoming a major problem because the track width has decreased due to advances in hard disk drive technologies, and because the write head element width has become closer to the read head element width. The combination of these two factors causes the tolerances for head drifting to decrease exponentially. To control the head drift, a significant waiting time may be required after the head steps from one track to the next, for the scanner to settle down to a stable position. Therefore, in this type of prior art certification testing, particularly where speed and cost are significant factors, as few as 100 to 200 tracks are tested on a disk containing thousands of tracks. Certification is then based on a small percentage of tracks tested and statistical methods are utilized to determine whether the number of defects detected will constitute rejection or acceptance of a disk.
The second type of disk certifier operates in a spiral-scanning mode wherein two heads are located on the disk apart from each other. One head performs the writing operation and the other performs the reading operation. Two scanners, two controllers and two electronic signal processing systems are needed for the scanning of the two heads. The advantage of this type of tester is that it can theoretically achieve very high efficiency (e.g., one revolution/track is possible). However, the major drawbacks of this type of disk certification tester are that since the two heads are located rather far from each other, stringent location-alignment precision must be maintained between the two heads in order for the read head to always read on-track with the write head, and a scanning speed equivalency must be maintained between the two scanners during scanning. Furthermore, due to the two head element arrangement, once the tester is set up for a specific configuration, it is difficult to change the configuration parameters (rpm, scanning speed, number of tracks tested, etc.) should the need arise. It is believed that these are among the reasons that many media manufacturers currently use the track-to-track stepping type testers.